The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of a rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor is characterized by a magnetiresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer). A pinning layer in a bottom spin valve is typically made of platinum manganese (PtMn). The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Sensors can also be categorized as current in plane (CIP) sensors or as current perpendicular to plane (CPP) sensors. In a CIP sensor, current flows from one side of the sensor to the other side in the parallel to the planes of the materials making up the sensor. Conversely, in a CPP sensor the sense current flows from the top of the sensor to the bottom of the sensor perpendicular to the plane of the layers of material making up the sensor.
Yet another type of sensor, somewhat similar to a CPP GMR sensor is a Tunnel Valve. A tunnel valve employs an electrically insulating spacer layer rather than a conductive spacer layer. A tunnel valve operates based on quantum mechanical tunneling of electrons through the insulating spacer layer. This tunneling is maximized when the magnetizations of the free and pinned layers are parallel to one another adjacent to the spacer layer.
Recently, researchers have found that a change in the material of the pinned layer adjacent to the spacer layer can increase δr of the sensor. For example it has been found if the magnetic layers of the pinned layer are comprised of substantially equal parts Co and Fe, ie. Co50Fe50, the δr of a CPP spin valve can be improved significantly. In a CIP spin valve similar δr improvement has been found with the use of Co90Fe10 in the pinned layer. These materials have strong positive magnetostriction, which means that the compressive stresses which inevitably occur in a spin valve will tend to magnetize the pinned layers perpendicular to the ABS of the sensor. This is not a problem for the pinned layer, and is even an advantage, since this is the desired direction of pinning, and the magnetostriction only acts to assist the desired pinning.
It would be possible to achieve a similar δr improvement by using similar material in the free layer (ie. Co50Fe50 for CPP, Co90Fe10 for CIP). However, the strong positive magnetostriction of these materials would magnetize the free layer in an undesirable direction perpendicular to the ABS. This would lead to unacceptable signal noise and free layer instability.
Another mechanism for increasing GMR effect, or δr, is to increase the thickness of the free and pinned layers. This is especially suitable in a CPP sensor where the total thickness of the sensor is not as limiting. It is known that increasing the thickness of the free layer can increase the GMR effect. However, increasing the free layer thickness also increases the magnetic thickness. This leads to free layer stiffness, because the coercivity of the free layer increases to the point that the sensor becomes insensitive to signals. One method that has been proposed to overcome this has been to form an antiparallel coupled free layer also referred to as a synthetic free layer. Such a synthetic free layer is similar to an AP pinned layer in that it has first and second magnetic layers having magnetizations that are antiparallel to one another across a coupling layer such as Ru. The synthetic free layer has a larger physical thickness than a simple free layer, but has a much smaller magnetic thickness, which is the difference between the magnetic thicknesses of the first and second magnetic layers.
A serious disadvantage of such a synthetic free layer is that the second magnetic layer (that which is furthest from the spacer layer) subtracts from the GMR effect, because it is 180 degrees out of phase with the magnetization of the first magnetic layer adjacent to the spacer. Therefore, any GMR advantage achieved by the use of synthetic free layers is essentially lost by this subtractive effect of the second layer.
Therefore, there remains a strong felt need for a mechanism for taking advantage of the increased GMR effects provided by the use of materials such as Co50Fe50 or Co90Fe10 in a free layer while mitigating the problems associated with the strong positive magnetostriction of such materials. There also remains a strong felt need for a means for utilizing the advantages of synthetic free layers in a spin valve without experiencing the subtractive effect of the second free layer on the GMR on the sensor.